The present invention relates to a compound or a salt thereof useful as an agent for measurement of singlet oxygen. The present invention also relates to an agent for measurement of singlet oxygen comprising the aforementioned compound or a salt thereof.
It is known that, in living bodies and life phenomena, free radical species such as nitrogen monoxide are acting as a second messenger for signal transduction, and they exert various physiological functions, for example, control of blood pressure in the circulatory system and the like. It has also been shown that superoxides and hydrogen peroxide as active oxygen species also exert important physiological functions in the immune system and the like. However, importance of singlet oxygen as a physiologically active species, which has an analogous electronic structure, has little been elucidated so far.
Recently, singlet oxygen has been revealed to be a reactive species of photodynamic therapy, which is one of cancer therapies, and it has been suggested that various kinds of oxidases, peroxidases and the like are generating singlet oxygen in living bodies. Furthermore, it has also been revealed that oxygen molecules act as a sensor and show signal-like actions, and therefore, singlet oxygen is also suggested to have possible responsibility of important physiological functions in living bodies.
Ten or more different methods are conventionally known as methods for measurement of singlet oxygen in living bodies, which include the chemiluminescence method, the electron spin resonance (ESR) method, the luminescence method and the like. However, these methods in common give only low specificity and sensitivity, and thus they are not reliable methods (as for the method for specific detection of singlet oxygen, see, Nagano, T., et al., Free radicals in Clinical Medicine, Vol. 7, pp.35-41, 1993, etc.). Therefore, it is desired to develop a method for measurement of singlet oxygen superior in specificity and sensitivity to study the involvement of singlet oxygen in life phenomena.
An object of the present invention is to provide a compound useful as an agent for measurement of singlet oxygen. Another object of the present invention is to provide an agent for measurement of singlet oxygen comprising said compound and a method for measurement of singlet oxygen using said compound. In particular, it is an object of the present invention to provide an agent for accurate measurement of singlet oxygen localized in particular cells or tissues in living bodies by a bioimaging technique.
The inventors of the present invention conducted various studies to achieve the foregoing objects. As a result, they found that a substantially non-fluorescent compound represented by the following general formula (I) efficiently reacts with singlet oxygen to give a fluorescent compound represented by the general formula (II). They also found that singlet oxygen can be measured with extremely high specificity and sensitivity by using a compound represented by the general formula (I) as an agent for measurement of singlet oxygen, and measuring fluorescence of a compound of the general formula (II) which is produced as a result of reaction of the compound represented by the general formula (I) and singlet oxygen localized in living cells or tissues. The present invention was achieved on the basis of these findings. 
wherein R1, R2, R3, R4, R5 and R6 independently represent a hydrogen atom, a halogen atom, a C1-6 alkyl group, or a C1-6 alkoxyl group, R7 and R8 independently represent a C1-6 alkyl group or an aryl group which may be substituted, R9 and R10 independently represent a hydrogen atom, a C1-6 alkyl group, or a C1-6 alkoxyl group, and R11 represents a hydrogen atom or a C1-12 alkanoyl group, or salts thereof.
From another aspect of the present invention, there are also provided compounds represented by the following general formula (II): 
wherein R12, R13, R14, R15, R16 and R17 independently represent a hydrogen atom, a halogen atom, a C1-6 alkyl group, or a C1-6 alkoxyl group, R18 and R19 independently represent a C1-6 alkyl group or an aryl group which may be substituted, R20 and R21 independently represent a hydrogen atom, a C1-6 alkyl group, or a C1-6 alkoxyl group, and R22 represents a hydrogen atom or a C1-12 alkanoyl group, or salts thereof.
From further aspects of the present invention, there are provided agents for measurement of singlet oxygen comprising a compound represented by the aforementioned formula (I) or a salt thereof; and methods for measuring singlet oxygen, which comprise the steps of: (A) reacting a compound of the aforementioned formula (I) or a salt thereof with singlet oxygen, and (B) measuring fluorescence of a compound of the aforementioned formula (II) or a salt thereof produced in the above step (A).
In addition to the above, there are also provided compounds represented by the following general formula (III): 
wherein R23, R24, R25, R26, R27 and R28 independently represent a hydrogen atom, a halogen atom, a C1-6 alkyl group, or a C1-6 alkoxyl group, R29 and R30 independently represent a C1-6 alkyl group or an aryl group which may be substituted, R31 and R32 independently represent a hydrogen atom, a C1-6 alkyl group or a C1-6 alkoxyl group, and R33 and R34 independently represent a C1-12 alkanoyl group, and compounds represented by the following general formula (IV): 
wherein R35, R36, R37, R38, R39 and R40 independently represent a hydrogen atom, a halogen atom, a C1-6 alkyl group, or a C1-6 alkoxyl group, R41 and R42 independently represent a C1-6 alkyl group or an aryl group which may be substituted, R43 and R44 independently represent a hydrogen atom, a C1-6 alkyl group or a C1-6 alkoxyl group, and R45 and R46 independently represents a C1-12 alkanoyl group. The compounds represented by the formula (III) are also useful as agents for measurement of singlet oxygen.